PhaST: Hierarchical Concurrent Log-Free Skip List for Persistent Memory

Skip list (skiplist) is a competitive index structure that offers superior concurrency and excellent performance but with high memory overhead and low access locality. Emerging persistent memory (PM) technologies present an opportunity to mitigate the capacity constraint of DRAM. However, data consistency on PM typically results in excessive write overhead. In addition, fast concurrent access to an index is critical to the throughput on high-end contemporary computer systems. In this article, we propose a Partitioned HierArchical SkiplisT called PhaST , which can simultaneously reduce the skiplist height and improve its access locality, through its hierarchy of component structures, while enabling fast parallel recovery in case of failure. To ensure high concurrency and fast data consistency, we also have developed writelock-free concurrent insert and log-free atomic split. Furthermore, we have developed a durable lock-free concurrent search that can discern transient structural inconsistencies and deliver highly concurrent read operations. We have conducted an extensive evaluation of PhaST compared to state-of-the-art studies such as NV-Skiplist, wB+-Tree, FPTree, and FAST-FAIR. Our evaluation results show PhaST outperforms other indexing structures by up to 4.05× and 2.87× in single-threaded inserts and searches, and 1.56× and 2.62× in concurrent inserts and searches.